It is known that a superconducting wire having a higher critical temperature can be formed by using a Bi (bismuth)- or Y (yttrium)-based oxide superconducting material than by using a metallic superconducting material such as Nb3Sn (niobium-tin alloy). In consequence, in recent years, it has been expected that a superconducting wire formed by using an oxide superconducting material, in particular, is applied as a wire for an electromagnet and power transmission. As described above, the superconducting wire in recent years can be used at a considerably higher temperature than does the conventional superconducting wire. Nevertheless, a superconducting coil or the like that is formed by using the above-described superconducting wire and that is placed in a cryogenic environment obtained by using liquid nitrogen is required to be electrically connected to the outside load. To meet the above requirement, a structure is employed in which the superconducting coil is placed in a liquid nitrogen bath to be electrically connected to the outside load by using a metal member such as a lead electrode. A pipe for supplying the liquid nitrogen, which is a cooling medium, to the liquid nitrogen bath is connected to the liquid nitrogen bath that mounts in it the superconducting coil.
It is desirable that the liquid nitrogen bath be a vacuum insulated container in order to suppress heat from penetrating into the liquid nitrogen bath that houses the liquid nitrogen (and the superconducting coil) so that the inside of the liquid nitrogen bath can be maintained under a sufficiently cooled temperature condition for enabling the superconducting coil to function. In particular, a vacuum insulated container provided with a radiation-heat-blocking film can highly efficiently suppress heat penetration from outside. As the liquid nitrogen bath (the vacuum insulated container) for housing the superconducting coil, a housing container made of FRP (fiber-reinforced plastic) has been widely used. Because FRP has sufficient strength and is low in cost, it has been widely used as the material of a housing container. The providing of a radiation-heat-blocking film at the inside of the vacuum insulated container (the housing container) made of FRP can secure the thermal insulation, against the outside, of the liquid nitrogen bath housing the superconducting coil.
The above-described housing container made of FRP is provided with an opening through the FRP plate forming the wall of the container, and metal members such as the lead electrode and metal pipe are placed such that they pass though the opening. In order to firmly fix the wall of the housing container made of FRP and the metal member penetrating the wall of the housing container to each other, conventionally, the wall of the opening provided through the wall of the housing container made of FRP is provided with an internal thread, and the periphery of the metal member placed such that it passes through the opening is provided with an external thread, and then both are fixed to each other by screwing the external thread into the internal thread. To further increase the bonding strength between the two members, an adhesive is sometimes placed between the external thread and the internal thread.
In the above-described method of fixing the metal member to the housing container made of FRP, the adhesive is applied from the outside of the opening of the FRP plate before the screwing-in of the external-thread portion of the metal member. As a result, when mechanical or thermal stress is applied to the adhesive-bonded portion between the internal and external threads, the bonded portion is sometimes separated or cracks sometimes develop at the bonded portion. When the foregoing separation or cracks develop, the air may leak into the vacuum insulated container from outside. When such a leak occurs, the function of the superconducting coil may be impaired.
To solve the above-described problem, for example, the published Japanese patent application Tokukai 2008-218861 (Patent literature 1) has disclosed a method of producing an FRP cryostat having a structure in which a flange portion is provided both at a member provided with an internal thread (an internal-thread member) and at a member provided with an external thread (an external-thread member) and the flange portion is placed such that its surface is flush with the surface of the plate member (the wall) of the housing container made of FRP. The FRP cryostat stated in Patent literature 1 has a structure in which the plate member (the wall) of the housing container made of FRP is provided with a recessed portion into which the flange portion is fitted. This structure allows uniform application of the adhesive throughout the bonding surfaces of both the bottom surface of the recessed portion and the inner surface of the fitted flange portion. Consequently, this structure further increases the bonding strength between the external-thread member and internal-thread member. In other words, even when mechanical or thermal stress is applied to the bonded portion between the external- and internal-thread members, the development of separation and cracks at the bonded portion can be suppressed. As a result, the function of the superconducting coil can be secured.
In the FRP cryostat disclosed in Patent literature 1, even when different materials are used for forming the external-thread member provided with the flange portion and for forming the metal member, such as a lead electrode, bonded with the external-thread member, because the two members are fixed to each other only at one place of the joint insertion hole formed in the external-thread member, this structure can mitigate the concentration of the thermal stress at the fixed portion, the thermal stress being produced by the difference in thermal expansion (the difference in low-temperature shrinkage ratio) between the external-thread member and the metal member.